1. Field of the Invention
The present invention relates to an optical beam scanning system in which a light beam from a light source is reshaped through a first image-forming system and is then reflected by a rotary polygon mirror, to thereby scan a surface-to-be-scanned through a second image-forming system.
2. Description of the Prior Art
FIG. 1 is a perspective view of a conventional optical beam scanning system. In the conventional optical beam scanning system, as shown in FIG. 1, a light beam L.sub.1 from a light source 1 impinges, via a first image-forming system 2, on a rotary polygon mirror 3 which rotates about an axis of rotation 3b in a direction A as indicated by an arrow.
The first image-forming system 2 includes a collimating lens 21 and a cylindrical lens 22. The collimating lens 21, as shown in FIG. 2A, is formed by a concave lens 21a and a convex lens 21b joined to each other. The cylindrical lens 22 has a refracting power only along a direction of the axis of rotation 3b, that is, a direction Y. Thus, the light beam L.sub.1, which deverges from the light source 1 enters the collimating lens 21 where it is collimated to become a collimated beam L.sub.2. The collimated beam L.sub.2 enters the cylindrical lens 22 to become a converging beam L.sub.3 which converges onto a mirror surface 3a of the rotary polygon mirror 3 only with respect to the direction Y (FIG. 2A).
The light beam L.sub.3 is deflected by the rotary polygon mirror 3 to become a deflected beam L.sub.4. The deflected beam L.sub.4 is focused on a surface-to-be-scanned 5 through a second image-forming system 4 which is comprised of a scanning lens 41 and a cylindrical lens 42, the cylindrical lens 42 having a refracting power only along the direction of the axis of rotation 3b (i.e., direction Y).
For convenience of illustration, FIGS. 2A and 2B illustrate the optical axis of the optical beam scanning system extending along a direction Z even though in reality, the optical axis changes its direction.
In this structured optical system, the mirror surface 3a of the rotary polygon mirror 3 and the surface-to-be-scanned 5 are in conjugation with each other with respect to the direction Y as shown in FIG. 2A. As a result, facet errors related to the mirror surface 3a are corrected, which in turn guarantee a high quality image recorded on the surface-to-be-scanned 5.
By expanding a diameter .phi..sub.X (FIG. 2B) of the light beam L.sub.3 taken in the X direction, the light beam can be focused on the surface-to-be-scanned 5 at a high resolution with respect to the main scanning direction. In other words, excellent focusing is obtainable if the collimated beam L.sub.2 from the collimate lens 21 has a large beam diameter .phi. (=.phi..sub.X =.phi..sub.Y).
As the beam diameter .phi. of the light beam L.sub.2 increases, the first image-forming system 2 has a longer optical path length. This is because the numerical aperture NA.sub.p (i.e., an angle .phi..sub.p) at the rotary polygon mirror 3 side is preliminarily determined by the second image-forming system 4. The fact that the angle .phi..sub.p is preliminarily determined naturally implies that the cylindrical lens 22 must have a longer focal length f.sub.22 along the direction Y to enlarge the beam diameter .phi. of the light beam L.sub.2. In addition, the numerical aperture NA.sub.0 (i.e., an angle .phi..sub.0) at the light source 1 side is constant. This circumstance requires the collimating lens 21 to have a long focal length f.sub.21 if a larger diameter .phi. of the light beam L.sub.2 is desired.
Assume that an optical beam scanning system satisfies the following design conditions:
Beam Diameter .phi. of Light Beam L.sub.2 =19 mm; PA1 Numerical Aperture NA.sub.0 at Light Source Side=0.1; and PA1 Numerical Aperture NA.sub.p at Rotary Polygon Mirror Side=0.0035
In such an optical beam scanning system, the first image-forming system 2 may be designed so as to have lens data as given in Table 1, for example.
TABLE 1 ______________________________________ i r.sub.Xi r.sub.Yi d.sub.i n.sub.i ______________________________________ 1 319.575 319.575 1.5 1.609089 2 27.71 27.71 4.0 1.582509 3 -60.579 -60.579 25.0 4 .infin. 1337.626 3.0 1.492821 5 .infin. .infin. 2727.474 ______________________________________
For convenience of description, the disclosure uses the symbol "r.sub.X " to refer to a radius of curvature of the light beam taken along the X direction on the optical axis. Likewise, by the symbol "r.sub.Y " is denoted a radius of curvature of the light beam taken along the Y direction on the optical axis. In Table 1, the symbols r.sub.Xi and r.sub.Yi denote radii of curvature of the surfaces of the i-th lens (i=1 to 5) from the light source 1. The symbol d.sub.i is a surface-to-surface distance, taken on the optical axis, between the i-th (i=1 to 4) lens and an (i+1)-th lens from the light source 1. The reference denotes "d.sub.5 " a distance between the cylindrical lens 22 and the mirror surface 3a. The symbols n.sub.1, n.sub.2 and n.sub.3 are refractive indices of the concave lens 21a, the convex lens 21b and the cylindrical lens 22 at a wavelength of 780 nm, respectively. Like symbols will adhere to the descriptions hereinafter in explaining other embodiments, and therefore, will not be explained again.
In the first image-forming system 2 having such structure, the focal length f.sub.21 of the collimating lens 21 and the focal length f.sub.22 of the cylindrical lens 22 taken along the Y direction are: EQU f.sub.21 =95 mm EQU f.sub.22 =2714.284 mm
The distance d.sub.0 between the light source 1 and the object-side surface of the first lens from the light source 1 is: EQU d.sub.0 =91.923 mm
Therefore, the optical path length d of the first image-forming system 2 is as long as d.apprxeq.2852 mm.